1. Field of the Invention
The present invention relates to a dielectric ceramic composition and a method for manufacturing multilayered components using the same. More specifically, the present invention is directed to a dielectric ceramic composition having dielectric properties suitable for high frequency regions such as microwave and millimetric wave, a low dielectric loss, a large dielectric constant and temperature stability, in which the dielectric composition can be sintered at low temperature and thus simultaneously baked with metal electrodes, such as silver, copper, silver/palladium, so being applicable to electronic parts, including resonators, multilayered ceramic capacitors, filters, dielectric substrate materials for monolithic IC (MIC), and dielectric waveguide materials and the like, and a method for manufacturing multilayered components using the dielectric ceramic composition.
2. Description of the Prior Art
Along with great advances in electronic and communication technologies, the hardware for embodying them has also been miniaturized. This miniaturization is due in part to new stacking and chipping techniques. Recently, because of the development of communication means using microwave bands, such as mobile phones and satellite broadcasting, there is continuing pressure and demand in the market place for further miniaturization of dielectrics components.
An example of the components to which stacking techniques are applied is a capacitor. Examples of stacked type components for use in mobile communication terminals include filters, couplers, duplexers, oscillators and multichip modules (MCM).
The stacked type components are generally composed of multilayered dielectrics and inner electrodes which are fabricated by laminating a dielectric into a thin tape, printing an inner electrode onto the dielectric laminate, stacking many laminates and firing the stack.
Suitable materials for the composition of inner electrodes include silver, copper, nickel, palladium, platinum, gold and alloys thereof. The selection of inner electrode materials is determined based on sintering temperature and the properties of the ceramic dielectrics which are sintered therewith.
In this regard, a silver (Ag) electrode, exhibiting the lowest specific resistance (1.62xc3x9710xe2x88x924 xcexa9m) and being inexpensive, cannot be applied to ceramic dielectrics which must be sintered at 950xc2x0 C. or higher, because of silver""s low melting point (mp.) of 961xc2x0 C. Meanwhile, gold (Au), platinum (Pt) or palladium (Pd) have low melting points, but are disadvantageous in terms of their high cost, so that the use of these materials is restricted. As for copper (Cu) or nickel (Ni) electrodes, their very poor oxidation resistance requires sintering at an oxygen partial pressure as low as about 10xe2x88x929 atm. The low pressure associated with thermally treating Nickel or copper electrodes under low oxygen, partial pressure causes most dielectric ceramic compositions to show highly increased dielectric loss, and therefore cannot be used as capacitors.
To be useful for stacked type components, dielectrics must be capable of being sintered along with electrodes in addition to having dielectric properties suitable for application. Such dielectric requirements include: 1) A high dielectric constant (∈r); 2) A quality factor (Qxf); and 3) A low dependency of resonance frequency modulation (xcfx84f) as a function of temperature change. Typically, these dielectric compositions are composed of BaOxe2x80x94TiO2, BaOxe2x80x94ReOxe2x80x94TiO2 (ReO is oxide of rare-earth elements), MgTiO3xe2x80x94CaTiO3. BaOxe2x80x94TiO2 has a high dielectric constant of 37-40, and a large quality factor (Qxf) of 30,000. However, in single-phase, BaOxe2x80x94TiO2 is not capable of achieving a temperature coefficient corresponding to a resonant frequency of zero. Further problems include the fact that the dielectric constant is changed according to the composition, and further, there is a high temperature dependency of the dielectric constant modulation. Because of these inherent properties, the temperature coefficient of resonant frequency of low values cannot be stably maintained, while keeping a high dielectric constant and low dielectric loss.
On the other hand, BaOxe2x80x94ReOxe2x80x94TiO2 types, including BaOxe2x80x94Nd2O3xe2x80x94TiO2 or BaOxe2x80x94Sm2O3xe2x80x94TiO2, have a high dielectric constant of 50-80, and stable temperature coefficients of resonant frequency, but are disadvantageous based on low Qxf values of 5,000 or lower. In MgTiO3xe2x80x94CaTiO3, the value of Qxf is 30,000 or higher, the temperature coefficient of resonant frequency is approximately zero, the dielectric constant is within the range of 16-25.
Ceramic dielectric compositions in current use in stacked type components are, for the most part, composed primarily of BaTiO3, by itself or in composition with oxide-sintering aids, such as Bi2O3, or glass frits, for decreasing sintering temperatures. Typically, the sintering of temperatures these conventional dielectric compositions range from about 1,100 to about 1,300xc2x0 C. Further, these dielectric compositions are resistant to reduction and possess dielectric constants with values of several hundred or higher. Their great dielectric loss, however makes it difficult to apply them in designs where a frequency band of 1 MHz or higher is used. Additionally, the dielectric compositions suffer from the drawback of undergoing a dielectric constant fluctuation of as large as several hundred ppm/xc2x0 C., which renders them unusable as temperature-stable capacitors or components for mobile communication devices.
Dielectric compositions known to be usable for stacked type components, operable with frequencies of 1 MHz or higher, are exemplified by CuO or V2O5-added Bi2O3xe2x80x94CaOxe2x80x94Nb2O5 and glass-added (Mg, Ca)TiO3, (Zr, Sn)TiO4 or (CaOxe2x80x94ZrO2). CuO or V2O5 added Bi2O3xe2x80x94CaOxe2x80x94Nb2O5 compositions may be sintered at 900xc2x0 C., possess a dielectric constant of 40 or higher, and a quality factor of 18,000 or higher. In addition, Japanese Laid-Open Patent Application No. Hei. 11-34231 and U.S. Pat. No. 5,350,639 relate to the manufacture of chip type stacked capacitors making use of low melting point electrodes including Ag and Cu, and dielectric resonators using strip lines.
Japanese Laid-Open Patent Application No. Hei. 9-315859 discloses that CaOxe2x80x94ZrO2 is added with alkaline earth-metal compounds including boron (B), lithium (Li) and sodium (Na), and thus can be sintered at low temperatures in the range of about 900xc2x0 to about 1,200xc2x0 C. These compositions, however, cannot be effectively sintered at a temperature of 1,000xc2x0 C. or lower, possess poor dielectric properties at microwave frequencies, and exhibit excess reactivity with electrode materials.
It is, accordingly, an abject of the present invention to provide a dielectric ceramic composition, which possesses excellent temperature stability, low temperature sintering properties, and a high dielectric quality factor at low dielectric bands.
Another object of the present invention is to provide a method for manufacturing multilayered components using the dielectric composition.
In one aspect of the present invention, there is provided a dielectric ceramic composition comprising (A) at least one oxide selected from the group consisting of La2O3, Nd2O3, Al2O3 and Y2O3, and (B) at least one oxide selected from the group consisting of Nb2O5, Ta2O5, at a molar ratio of 1:1, and being represented by the general formula:
Axe2x80x2xAxe2x80x31xe2x88x92xBxe2x80x2yBxe2x80x31xe2x88x92yO4
(wherein, Axe2x80x2 and Axe2x80x3 represent elements selected from the group consisting of La, Nd, Y and Al; Bxe2x80x2 and Bxe2x80x3 represent elements selected from the group consisting of Nb and Ta; and x and y represent mole fractions, each in the range of 0xe2x89xa6xxe2x89xa61.0 and 0xe2x89xa6yxe2x89xa61.0, respectively).
As such, the dielectric ceramic composition may further comprise at least one sintering aid selected from the group consisting of B2O3, CuO, ZnO and Bi2O3, and at least one additive selected from the group consisting of V2O5, SnO2, MgO, NiO, Sb2O3, LiF and Ag2O.
In another aspect of the present invention, there is provided a method for manufacturing multilayered components using the dielectric ceramic composition, comprising the following steps of: mixing a main component represented by the formula Axe2x80x2xAxe2x80x31xe2x88x92xBxe2x80x2yBxe2x80x31xe2x88x92yO4 (wherein Axe2x80x2 and Axe2x80x3 are selected from the group consisting of La, Nd, Y and Al; Bxe2x80x2 and Bxe2x80x3 are selected from the group consisting of Nb and Ta; and x and y are mole fractions, each in the range of 0xe2x89xa6xxe2x89xa61.0 and 0xe2x89xa6yxe2x89xa61.0, respectively) with a sintering aid, an additive, or mixtures thereof, to form a slurry,; deaerating the slurry, molding the deaerating slurry into a tape form; printing an inner electrode on the molded tape by use of a low melting point electrode paste characterized by a melting point of about 1000xc2x0 C. or lower; laminating the inner electrode-printed tape to at least two layers; and sintering the laminated tape in a sintering furnace at about 1000xc2x0 C. or lower.
A LaNbO4 composition, based on the formula Axe2x80x2xAxe2x80x31xe2x88x92xBxe2x80x2yBxe2x80x31xe2x88x92yO4 composition of the present invention, is not reported for low temperature sintering and dielectric properties at microwave bands, except for monoclinic-to-tetragonal phase transformation (Jian et al., J. Am. Ceram. Soc., 80(3) 803-806 (1997)).
To improve the dielectric properties of the LaNbO4 composition, a cationic substituent selected from the group consisting of Nd2O3, Al2O3, Y2O3, Bi2O3, Sb2O3, Ta2O5 and mixtures thereof may be added to the composition. As a result of adding a cationic substituent to the LaNbO4-based composition, the dielectric constant and temperature coefficient of resonant frequency of the composition become substantially improved. The cationic substituents, Nd2O3, Al2O3, Y2O3, Bi2O3 and Sb2O3 function by substituting Nd, Al, Y, Bi and Sb for La of the main composition. As such, a preferable amount of a substituent corresponding to Nd2O3, Al2O3, Y2O3, Bi2O3 and Sb2O3 falls within the range of from about 0.01 to about 50 mole %. If the substituent amount is outside said range, the dielectric loss and temperature coefficient are increased. On the other hand, Ta2O5 is used to substitute Ta for Nb of the LaNbO4-based composition. Furthermore, the substituent Ta2O5 is favorably added in the amount ranging from about 0.01 to about 50 mole %. The substituent amount beyond said range results in poor sintering properties.
The Axe2x80x2xAxe2x80x31xe2x88x92xBxe2x80x2yBxe2x80x31xe2x88x92yO4 based compositions, such as the LaNbO4-based and cation-substituted compositions, are added to a mixture with at least one oxide, as a sintering aid selected from the group consisting of B2O3, CuO, ZnO and Bi2O3, and subsequently mixed. The solvent-removed mixture is calcined, ground, mixed in the presence of a binder, molded, and sintered, to yield a dielectric composition. The sintering aid is preferably added in an amount ranging from about 0.01 to about 7 parts by weight of the main composition. If the added amount of the sintering aid meets said range, sintering of the composition is enhanced, and the dielectric properties of the composition are simultaneously improved. However, when the amount of the sintering aid is beyond said range, no improvements can be expected in either the sintering or the dielectric properties of the composition.
It is necessary to add at least one additive selected from the group consisting of V2O5, SnO2, MgO, NiO, Sb2O3, LiF and Ag2O, together with the sintering aid. The additive is preferably used in an amount ranging from about 0.01 to about 7 parts by weight on the basis of 100 parts by weight of the main composition. Within this range, the additive acts to improve the sintering properties of the main composition.
In the present invention, the sintering aid is added to lower the sintering temperature of the Axe2x80x2xAxe2x80x31xe2x88x92xBxe2x80x2yBxe2x80x31xe2x88x92yO4-based composition to a temperature ranging from about 850-1,000xc2x0 C. The method for preparing the dielectric composition of the present invention can be used in the fabrication of practical lamination parts, such as chip LC filters, chip duplexers, and dielectric filters for personal communication service (PCS). As explained great detail below, the method of preparing multilayered components, such as dielectric filters for PCS, is accomplished by adding at least one cationic substituent to a LaNbO4-based composition and selectively adding the sintering aid or the additive to the mixture.
The starting material corresponding to the present dielectric composition is weighed, and added with polyvinyl butyral and a plasticizer. The mixture is then introduced into an organic solvent and mixed for 24 hours in order to prepare a slurry for tape casting.
The slurry is subsequently deaerated and molded using a tape caster into a thin dielectric tape with a thickness ranging from about 10 to about 150 xcexcm. An inner electrode is then printed on the molded tape using a low melting point electrode paste with a melting point of about 1,000xc2x0 C. or lower melting point.
Subsequently, the inner electrode-printed tape is laminated in at least two layers, and then the laminated tape is sintered in a sintering furnace at a temperature of about 1,000xc2x0 C. or lower.
Alternatively, dielectric powders may be used to prepare a paste, which is then repeatedly printed resulting in the manufacturing of multilayered components.
The dielectric composition of the present invention can be sintered at about 900xc2x0 C., thus being capable of simultaneous sintering with low melting point electrodes, such as pure silver (Ag). Additionally, the composition has the temperature coefficient of resonant frequency of about xc2x110 ppm/xc2x0 C. or lower, and can be applied to temperature stability-requiring parts, for instance, temperature-stable lamination capacitors (NPO MLCC). In addition, the dielectric composition is characterized by a superior quality factor (Qxf) of 40,000 or more at frequency bands of 8-11 GHz. Therefore, the dielectric composition can be used for mobile communication parts including microwave filters, oscillators, planar antenna, MCM and the like. The dielectric properties of said dielectric composition are hardly changed at the sintering temperature range of about 900 to about 950xc2x0 C., whereby the composition with a temperature coefficient of resonant frequency (xcfx84f) of xc2x110 ppm/xc2x0 C. or lower can be stably used in products manufacturing.
The present invention is further illustrated by the following examples, which are not to be construed in any way as imposing limitations upon the invention""s scope. On the contrary, it is to be clearly understood that resort may be had to various other embodiments, modifications, and equivalents thereof which, after reading the description herein, may suggest themselves to those skilled in the art without departing from the spirit of the present invention and/or the scope of the appended claims.